Various forms of semi-trailers designed to carry heavy loads are provided with central lift wheel assemblies, such as that disclosed in U.S. Pat. No. 3,093,388 wherein the lift wheel assembly may be used, during over-the-road operations, for supporting a portion of the load and with the lift wheel assembly being upwardly retractable when the trailer is moving at slow speeds and executing sharp turns. However, this type of lift wheel assembly can experience excessive lateral forces and sliding tire wear if the lift wheel assembly is not lifted during sharp turns and backing operations. Further, although various forms of trucks and other wheeled load supporting structures include caster wheel assemblies which are freely pivotable about upstanding axes such as those disclosed in U.S. Pat. Nos. 3,246,908 and 1,672,035, the utilization of full caster wheel assemblies on high speed vehicles has not been carried out due to the tendency of such caster wheel assemblies to shimmy at high speeds.